[2.2] paracyclophane and derivatives thereof are dimers which can be represented by the following formula: ##STR1## wherein B is typically hydrogen, a halogen, an alkyl, an aralkyl, or a halogen-aralkyl radical containing from 1 to about 20 carbon atoms, and m is zero or an integer from 1 to 4.
[2.2] paracyclophane (hereinafter referred to as "PCP") and its derivatives are commonly used as starting materials in the preparation of poly-p-xylylenes (hereinafter referred to as "parylene"). Parylene is often used to provide conformal coatings on a variety of substrates, such as, for example, circuit boards and electronic components, medical implant devices and surgical equipment. Typically, parylene is applied to the particular substrate by a vacuum vapor deposition technique wherein the PCP, or derivative thereof, is vaporized at an elevated temperature and polymerized directly onto the substrate to form a conformal coating of parylene.
PCP is typically prepared by reacting p-methylbenzyltrimethylammonium hydroxide (hereinafter referred to as "Q-hydroxide") in an alkaline medium via the Hofmann elimination reaction, the details of which are known to those skilled in the art. Typically, the Q-hydroxide is formed in situ by the reaction of a p-methylbenzyltrimethylammonium halide (hereinafter referred to as "Q-salt") with the alkaline medium. Thus, Q-salt is a common starting material in the preparation of PCP. Derivatives of PCP can be prepared, for example, by conducting the Hofmann elimination reaction on a corresponding derivative of Q-salt.
One problem often encountered in the preparation of PCP and derivatives thereof is that undesired products, such as trimers, other oligomers and higher polymers, can be produced as by-products (hereinafter referred to as "by-product polymers"). It is not uncommon for the yield of such by-product polymers to be greater than 5% and often greater than 10%. The term "yield" as used herein means the yield based on the stoichiometric conversion of two moles of Q-hydroxide forming one mole of PCP. Thus the yield is equal to the moles of reaction product divided by twice the moles of Q-salt or Q-hydroxide used as feed in the reaction multiplied by 100. Aside from detracting from the yield of the desired product, i.e., PCP and derivatives thereof, the formation of by-product polymers is additionally undesirable because such by-product polymers often assume a gelatinous appearance and can be difficult to separate from the reaction product.
Chemical inhibitors, such as, for example, dibenzothiazine (hereinafter referred to as "phenothiazine") have been used to inhibit polymer formation during the preparation of PCP and derivatives thereof. As used herein, the term "chemical inhibitor" means any chemical which is added to the reaction in order to inhibit the formation of by-product polymer. Other commonly used chemical inhibitors include, for example, buytlated hydroxytoluene (commonly known as "BHT"), hydroquinone and derivatives thereof. When chemical inhibitors such as described above are employed, trace amounts of the inhibitors are often present in the desired product. Trace amounts of phenothiazine can cause discoloration of the product and also can interfere with subsequent processing steps, e.g., in the chlorination of PCP. Accordingly, it is often necessary to perform purification steps in order to remove such chemical inhibitors from the PCP and derivatives thereof.
Therefore, improved processes are desired for the preparation of PCP and derivatives thereof which can inhibit the formation of by-product polymers without requiring the use of chemical inhibitors.